[RFC Home] [TEXT|PDF|HTML] [Tracker] [IPR] [Info page]

Obsoleted by: 3802 PROPOSED STANDARD
Network Working Group                                         G. Vaudreuil
Request for Comments: 2422                             Lucent Technologies
Obsoletes: 1911                                                 G. Parsons
Category: Standards Track                                 Northern Telecom
                                                            September 1998

                  Toll Quality Voice - 32 kbit/s ADPCM
                       MIME Sub-type Registration

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.


   This document describes the registration of the MIME sub-type
   audio/32KADPCM for toll quality audio.  This audio encoding is
   defined by the ITU-T in Recommendation G.726.

1.  Abstract

   This document describes the registration of the MIME sub-type
   audio/32KADPCM for toll quality audio.  This audio encoding is
   defined by the ITU-T in Recommendation G.726.  This document refines
   an earlier sub-type registration in RFC 1911.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [REQ].

2. ITU-T Definition

   Recommendation G.726 [G726] defines the characteristics that are
   recommended for the conversion of a 64 kbit/s A-law or m-law pulse
   code modulation (PCM) channel at 8000 samples/second to and from a
   40, 32, 24 or 16 kbit/s channel. The conversion is applied to the PCM

Vaudreuil & Parsons         Standards Track                     [Page 1]

RFC 2422                    32 kbit/s ADPCM               September 1998

   bit stream using an adaptive differential pulse code modulation
   (ADPCM) transcoding technique.  This Recommendation obsoletes G.721
   which only defined the 32 kbit/s characteristics.

   Recommendation G.726 was prepared by Study Group 15 of the
   Telecommunications Standardization Sector of the International
   Telecommunication Union (ITU-T) and was approved under the ITU's
   Resolution No. 2 procedure on the 14 of December 1990.

3. MIME Definition

3.1 audio/32KADPCM

   CCITT Recommendation G.726 [G726] describes the algorithm recommended
   for conversion of a 64 kbit/s A-law or u-law PCM channel to and from
   a 32 kbit/s channel (this is the same algorithm as described in the
   deprecated G.721).  The conversion is applied to the PCM stream using
   an Adaptive Differential Pulse Code Modulation (ADPCM) transcoding

   The MIME sub-type audio/32KADPCM is defined to hold binary audio data
   encoded in 32 kbit/s ADPCM exactly as defined by ITU-T Recommendation
   G.726.  No header information shall be included as part of the audio
   data.  The content transfer encoding is typically either binary or

   An additional consideration that this document defines for clarity is
   the choice of little endian ordering of the four bit code words.
   This default ordering is defined in ITU-T Recommendation X.420 [X420]
   for the equivalent X.400 body part, but is also detailed below in the
   IANA Registration.

3.2 VPIM Usage

   The audio/32KADPCM sub-type is a primary component of the VPIM
   specification [VPIM].  In this context, the Content-Description and
   Content-Disposition headers are used to succinctly describe the
   contents of the audio body.  As well, only the little endian bit
   ordering is valid.  Refer to the VPIM Specifcation for proper usage.

Vaudreuil & Parsons         Standards Track                     [Page 2]

RFC 2422                    32 kbit/s ADPCM               September 1998

4.  IANA Registration

   To: ietf-types@iana.org
   Subject: Registration of MIME media type audio/32KADPCM

   MIME media type name: audio

   MIME subtype name: 32KADPCM

   Required parameters: none

   Optional parameters: none

   Encoding considerations:

      Binary or Base-64 generally preferred

   Security considerations:

      There are no known security risks with the sending or
      playing of raw audio data  Audio data is typically
      interpreted only by an audio codec.  Unintended information
      introduced into the data stream will result in noise.

   Interoperability considerations:

      The four bit code word ordering within a byte may differ
      between existing implementations of G.726 codecs.  Since
      this content only permits the little endian ordering, codecs
      that support the opposite ordering must reorder the code
      words before storing to or retrieving from this content

   Published specification:

           ITU-T G.726 with little endian ordering

   Applications which use this media type:

           primarily voice messaging

   Additional information:

     Magic number(s): ?
     File extension(s): .726

Vaudreuil & Parsons         Standards Track                     [Page 3]

RFC 2422                    32 kbit/s ADPCM               September 1998

     Macintosh File Type Code(s):  APCM

      Little Endian Ordering:

      The 4-bit code words of the G.726 encoding MUST be packed
      into octets/bytes as follows:  the first code word (A) is
      placed in the four least significant bits of the first
      octet, with the least significant bit (LSB) of the code word
      (A0) in the least significant bit of the octet;  the second
      code word (B) is placed in the four most significant bits of
      the first octet, with the most significant bit (MSB) of the
      code word (B3) in the most significant bit of the octet.
      Subsequent pairs of the code words shall be packed in the
      same way into successive octets, with the first code word of
      each pair placed in the least significant four bits of the
      octet.  It is preferred that the voice sample be extended
      with silence such that the encoded value comprises an even
      number of code words.  However, if the voice sample
      comprises an odd number of code words, then the last code
      word shall be discarded.

      MSB ->  | 7| 6| 5| 4| 3| 2| 1| 0|  <- LSB

              32K ADPCM / Octet Mapping

   Person & email address to contact for further information:

     Glenn W. Parsons

     Gregory M. Vaudreuil

   Intended usage: COMMON

   Author/Change controller:

     Glenn W. Parsons & Gregory M. Vaudreuil

Vaudreuil & Parsons         Standards Track                     [Page 4]

RFC 2422                    32 kbit/s ADPCM               September 1998

5. Authors' Addresses

   Glenn W. Parsons
   Northern Telecom
   P.O. Box 3511, Station C
   Ottawa, ON  K1Y 4H7

   Phone: +1-613-763-7582
   Fax: +1-613-763-4461
   EMail: Glenn.Parsons@Nortel.ca

   Gregory M. Vaudreuil
   Lucent Technologies
   17080 Dallas Parkway
   Dallas, TX  75248-1905
   United States

   Phone/Fax: +1-972-733-2722

6. References

   [G726] CCITT Recommendation G.726 (1990), General Aspects of Digital
          Transmission Systems, Terminal Equipment - 40, 32, 24,16
          kbit/s Adaptive Differential Pulse Code Modulation (ADPCM).

   [MIME4] Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
           Mail Extensions (MIME) Part Four: Registration Procedures",
           RFC 2048, November 1996.

   [VPIM1] Vaudreuil, G., "Voice Profile for Internet Mail", RFC 1911,
           February 1996.

   [VPIM2] Vaudreuil, G., and G. Parsons, "Voice Profile for Internet
           Mail - version 2", RFC 2421, September 1998.

   [X420] ITU-T Recommendation X.420 (1996) - ISO/IEC 10021-7:1996,
          Message handling systems: Interpersonal messaging.

   [REQ] Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

Vaudreuil & Parsons         Standards Track                     [Page 5]

RFC 2422                    32 kbit/s ADPCM               September 1998

7.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implmentation may be prepared, copied, published and
   distributed, in whole or in part, without restriction of any kind,
   provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

Vaudreuil & Parsons         Standards Track                     [Page 6]